Different methods of wavefront sensing are known, psycho-physical, involving the human subject and objectives, such as refractive, laser ray tracing (LRT), Shack-Hartmann (SH) wavefront sensors (WFS), pyramid (P) wavefront sensors.
So far, all these methods provide 2D aberration information. Each method and associated device sample the transversal distribution of aberrations. LRT, by collecting a limited number of points in transversal section of the beam while moving the investigating beam parallel to itself before incidence on the cornea. To enhance the speed, an advanced LRT method is presented in U.S. Pat. No. 6,561,648 to D. Thomas, where more than one beam at a time is produced by a spatial light modulator (SLM) and used to provide a LRT type wavefront sampling. However, the method cannot provide depth resolved wavefront information due to the low numerical aperture associated to each beamlet created by the SLM.
Principles of SH/WFS are described in several prior publications, such as J. J. Widiker, S. R. Harris, and B. D. Duncan, “High-speed Shack-Hartmann wavefront sensor design with commercial off-the-shelf optics,” Appl. Opt. 45, 383-395 (2006) and A. Chernyshov, U. Sten, F. Riehle, J. Helmcke, and J. Pfund, “Calibration of a Shack-Hartmann sensor for absolute measurements of wavefronts,” Appl. Opt. 44, 6419-6425 (2005). A SH/WFS uses a limited number of lenses in a lenslet array and the beams traversing such micro-lenses have small diameters. A pyramid sensor uses a limited number of pixels according to the number of pixels in 2D array cameras. Again, due to the limited numerical aperture in each of the beams associated with microlenses or photodetectors, SH/WFS or P/WFS, the WFS has little sensitivity to depth in the object. The lenses in the SH/WFS sample a tiny part of the interrogated beam, for 10×10 number of lenses, less than 1/10th of the beam is intercepted by the micro-lens. Given their focal length, usually 1 mm-1 cm and their diameter, 0.5-5 mm, the confocal depth range of each measuring channel is hundreds of microns or millimeters. Each lens in the lenslet operates like a confocal microscope (CM) channel. Therefore, the deviation of the focused spot from the node of the grid corresponding to a non-aberrated beam for that lens represents an integration of aberrations over the depth range of the corresponding CM channel in the SH/WFS. This makes the SH/WFS insensitive to depth variations of aberrations, or more precisely, the spots are deviated from the ideal wavefront grid by quantities which represent averages of aberrations over the depth of focus of the confocal microscopy channel of each lens in the lenslet array. This depth of focus is comparable with the 1 mm tissue thickness of interest or much larger.
Systems are also known which combine WFSs with imaging configurations, such as those disclosed in U.S. Pat. No. 5,949,521, WO2003/105678 A2. All the WFSs presented and the systems using them have the disadvantage that for thick objects, the variation of aberration with depth is disregarded. The aberration introduced depends on the depth where the reflection originates from and such information is not acquired. Acquisition of depth resolved aberration is especially important in microscopy, where shallow layers deteriorate the curvature of the beam.
A general problem with prior art configurations is that they acquire aberration information from a large depth range of the object investigated. Their depth range is that determined by the focus elements and apertures within the sensor. For instance, when the sensor is a SH/WFS, each lens in the lenslet array together with all other parts in the optics interface between the lens and the object implements a CM channel with a very large depth of focus, sometimes larger than the tissue or the microscopy specimen examined.
As another problem of prior art, the depth resolved variation of aberration is ignored due to the principle of imaging involved. The paper “Adaptive optics parallel spectral domain optical coherence tomography for imaging the living retina” by Yan Zhang, Jungtae Rha, Ravi S. Jonnal, and Donald T. Miller, published in Opt. Express, Vol. 13, No. 12, pages 4792-4811 presents a combination of a SH-WFS controlling a deformable mirror, with a spectral domain OCT (SD-OCT) camera based on a free-space parallel illumination architecture. A B-scan (cross-section) of the retina is obtained, but for points along A-scans in the image, there is no alteration of the correction to take into account the aberration variation with depth. No such information is acquired, while it is expected that the aberrations vary as the coherence gate of the OCT channel progresses in depth. On the other hand, even if depth resolved aberration information was provided, the OCT method employed cannot be used in generating a corrected OCT image depending on the variation of aberrations with depth, as the SD-OCT is based on collecting A-scans under a fixed focus.
Due to the reasons mentioned above, an average correction is achieved only, based on the average of aberrations over the depth of range of the wavefront sensor.
There are also microscope specimens where the shallow layers distort the imaging of deeper layers.
As another disadvantage of prior art is that WFSs use sensitive photodetectors or arrays of photodetectors which are easily disturbed by stray reflections in the optics. For instance, reflections form lenses in the interface optics of microscopes and reflections from cornea affect the operation of the WFSs. These have to be eliminated in microscopes and in the OCT and SLO systems for imaging the eye. Therefore, spatial filters are used, which are not 100% functional, i.e. they do not eliminate the reflections from the different interfaces or from the cornea totally. Therefore, the cornea is placed off-axis, which introduces aberrations. This is achieved by working off-axis with one of the beams, either that of the WFS reference beam or the imaging beam, as disclosed in the U.S. Pat. No. 6,264,328 (Wavefront sensor with off-axis illumination). In this case, the beam used for the WFS cannot be shared by the imaging instrument, because the imaging beam has to cross the middle of the pupil at different angles.
The problem of stray reflections determines the use of single path correction configurations, where a thin beam is sent to the eye and aberrations are picked up by the emerging beam coming out of the eye. In this case, the correction cannot be applied dynamically, as described in the paper Adaptive-optics ultrahigh-resolution optical coherence tomography, by B. Hermann, et al, published in Optics Letters, Vol. 28, No. 18, 2004, 2142-2144. This article presents a flying spot system OCT, single path correction and sequential work of OCT and AO channels.
In double path, the same beam towards the object is shared by the WFS and the imaging instrument, in which case the on-axis corneal reflection saturates the CCD camera in the WFS as mentioned above.
Also, in double path correcting configurations of fundus cameras, SLOs or OCTs which use lenses between the scanners and the eye, the stray reflections from the lenses affect the WFSs. Therefore, curved mirrors are preferred to lenses with the disadvantage of increased set-up size and cost.
Therefore, better WFSs are needed, which can provide aberration information at each depth in the sample, either a thick specimen in microscopy or the retina.
WFSs are also needed, less sensitive to stray reflections in the optical configuration.
In addition, chromatic aberrations are often ignored. Previous WFS studies have used a series of filters to select the wavelength measurements and have performed the wavefront measurements one after another, as described in the article “Axial chromatic aberration of the human eye,” published by R. E. Bedford and G. Wyszecki in the Journal of the Optical Society of America 47, 564-565 (1957).
The system described in the article “Ocular aberrations as a function of wavelength in the near infrared measured with a femtosecond laser,” published by E. Fernandez, A. Unterhuber, P. Prieto, B. Hermann, W. Drexler, and P. Artal in Optics Express 13, 400-409 (2005) was used to show that defocus in the eye can change by up to 0.4 diopters in the human eye when the wavelength is changed within a 200 nm range in the IR range centered on 800 nm.
The evaluation of chromatic aberration is therefore essential for achieving good performance in high resolution imaging of the retina.
The article “Coherence-gated wave-front sensing in strongly scattering samples”, Marcus Feierabend, Markus Rückel, and Winfried Denk, published in Optics Letters, (2004) Vol. 29, No. 19, p. 2255-2254 and the US application 2006/0033933 disclose a method based on low coherence interferometry to produce 3D distribution of the scattered wave by analyzing its phase. The coherence gated (CG) information is then followed by sampling the 3D data into spatial arrays corresponding to SH apertures. Such apertures are virtual. The time to work out the interference signal is relatively high and such a procedure is subject to cross talk between pixels, which alters the phase information. These deficiencies restricted the development of the method. A lenslet array is also suggested instead of the virtual evaluation of electrical fields on sub-arrays, but this is placed in the object arm. This prevents using the same beam sent to the object for imaging. Imaging and wavefront sensing at the same time or with the same beam is not possible. In both cases, using a virtual lenslet or a real lenslet array, the method relies on phase calculations. The move from interferometric to Shack-Hartmann sensing was motivated by the need to avoid phase unwrapping and phase stability problems of the interfering sensors. The article and patent by Markus Rückel, and Winfried Denk mentioned above require phase unwrapping for large aberrations.
A major problem with the prior wavefront sensing technology, with or without coherence gating, is that the information is acquired and provided en-face. Progress in fast OCT imaging requires such information and potential correction to be provided in cross-sections and not in en-face orientation.
Thus, a need exists for addressing the problems of the prior art mentioned above using novel principles of WFS. Novel optical configurations are also needed which can employ such enhanced WFS in improving the imaging resolution.